The invention relates to a heating device for motor vehicles, in particular, a supplementary heating device that includes a working chamber filled with a viscous fluid and a rotor that rotates in the working chamber.
Such a heating device has been disclosed by DE-A-38 32 966. Unlike the heating device of the prior art, however, the invention is not restricted to motor vehicles with fluid-cooled internal combustion engines, but also has application in electric vehicles and vehicles with so-called hybrid drive, i.e. with a combined drive comprising an electric motor and internal combustion engine.
The known heating device according to DE-A-38 32 966, which corresponds to U.S. Pat. No. 4,974,778 issued Dec. 4, 1990 to Bertling (which is hereby incorporated by reference), discloses a supplementary heating device for fluid-cooled internal combustion engines which produces heat as a result of fluid friction and emits said heat to the cooling water of the internal combustion engine. The heat is produced by a pot-shaped rotor which is continuously driven by the internal combustion engine and rotates in a working chamber. The working chamber is only partially filled with a viscous medium. Next to this working chamber there is a reservoir chamber which is connected to the working chamber via an inflow port and a return port.
The prior-art heating device is regulated by means of a temperature-controlled valve which controls the flow of viscous medium from the reservoir chamber into the working chamber. If the temperature drops below a specific coolant temperature, the valve opens and viscous medium flows into the working chamber. If the temperature exceeds a specific coolant temperature, the valve closes and the viscous medium is pumped out of the working chamber. However, because of the heating device's configuration, it is not possible to pump all the oil out of the working chamber. As a result of the continuously running drive of the rotor, the remaining oil continues to produce a certain degree of residual heat which is passed on to the cooling water. This residual heat is not desired when the warming-up phase of the engine has finished. Likewise, this continuous idling, subject to friction, means a loss of power for the internal combustion engine.
Moreover, the function of this known heating device is questionable during operation of the device. Because the (dynamic) pressure in the working or friction gap is higher than the pressure, produced by the force of gravity, on the oil outside the working gap, the working gap (which produces the heat) cannot be filled continuously, especially since the oil is also not pumped away continuously. Therefore, when the valve is opened the oil only passes into the working gap in an uncontrolled manner, i.e. randomly, and therefore produces virtually no heat.
DE-A-31 47 468 discloses a further heating device which operates according to the principles of a hydrodynamic brake. The impeller of the brake is driven by the internal combustion engine via a V-belt drive. The brake is switched on and off via an electromagnetic clutch. The hydrodynamic brake has, inter alia, the disadvantage that the heating effect is relatively low at low speeds of the driving motor, which is disadvantageous particularly when the machine is idling in the warming-up phase.